I wasn't planning on putting screen inside the downspouts, but maybe inside the plenum areas on the ends of the collector since they aren't really capturing much heat otherwise.

A second question is about the intake and exhaust air piping. My plan is to run pipes through holes I am cutting into my crawlspace vents so I will draw air out of the crawlspace, heat it and then push it back in. Is there any advantage to using PVC pipes over standard sheet metal rigid dryer vent type pipes? Most of the designs I have seen online seem to be using 4 or 6 inch PVC pipes.

It should be pulled fairly tight so as to smooth the sides as much as possible. The goal is to keep the heat from dissipating before it entered the home. Bare ducts or pipes would lose a lot of heat before is enters the home. This is especially important if the duct runs are long or are in an unconditioned area of the home. When you say you want to draw air out of the crawlspace, is this crawlspace part of the actual conditioned part of the home? And are the crawlspace walls insulated?

You should be drawing intake air from the living space of the home, out through the heater, and back into the living space.

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